A bearing is generally classified into a rolling bearing (using a ball or a roller), an oilless bearing (using a lubricant material for a frictional operation), a sliding bearing (using an oil), a gas bearing, and a magnetic bearing (using magnetic force for a contactless operation). The sliding bearing is divided into a hydrodynamic sliding bearing and a hydrostatic sliding bearing. The hydrodynamic sliding bearing supports a shaft using an oil pressure generated by a relative sliding motion. The hydrostatic sliding bearing supports a shaft using high-pressure oil supplied from the exterior of the bearing. The gas bearing is operated in the same manner as in the sliding bearing, excepting that gas is used instead of oil. The hydrostatic gas bearing is supplied with a compressed gas from the external source, and in the hydrodynamic gas bearing the pressure is generated by a relative sliding motion.
The hydrodynamic gas bearing is widely used in the high-speed rotation applications, due to its low friction loss and unnecessity of liquid lubricant. In particular, it is used commonly in case of superspeed applications where the rolling bearing cannot be used for supporting and in case where a liquid lubricant cannot be easily used. The hydrodynamic gas bearing is categorized into a grooved bearing, a tilting pad bearing, and a foil bearing. The grooved bearing has a groove for generating a pressure, and exemplified by a spiral grooved bearing. In the case the hydrodynamic fluid-film tilt pad bearing, its working condition is very restricted and thus a risk of failure is increased disadvantageously if beyond the working condition. For example, since the rigidity thereof is rapidly decreased when above or below the design criteria, this bearing is very susceptible to impact, misalignment of a shaft, and thermal deformation. In contrast, a foil bearing called a compliant hydrodynamic fluid-film bearing provide a very high performance, and a rapid progress has been made recent 20 years. In addition, its adequate durability and stability has been confirmed in the air conditioning device of airplanes. In particular, it has been employed in a high-speed rotation machine such as a high-speed cryogenic turbo-compressor of 100,000 rpm. This bearing can be used with minute liquid mixed and its flexibility and the possibility of lower price are their advantages. The foil bearing for airplanes has been used mainly since 1970 in the air cooling machine (ACM), which is a core component for controlling the temperature and pressure inside the cabin in the environmental control system (ESC). This can be considered as a most suitable example of use. In this application, the foil bearing does not contaminate the interior of the cabin because it does not have any oil system. Also, it has enabled a stable operation for a long time, without scheduled maintenance, as compared with a ball bearing. When failed, advantageously it does not lead to the failure of other turbo-components. The foil bearing used in Boeing 747 has been being operated more than 100,000 hours, without any repair.